The principle of operation of a highly precise and inherent stable analyzer system for a multipole mass filter, which is generally known as a multipole mass filter according to Paul, is described in the German Pat. No. 944,900.
A multipole usually consists of electrically conductive round or hyperbolic rods, the number of rods corresponding to the number of poles. A quadrupole, in particular, consists of four parallel electrically conductive round or hyperbolic rods. The rods are held in a parallel position relative to each other by means of one or several electrically insulating mounting members in the form of rings or cages which embrace the rods on the outside. The centers of the rods are arranged in a square when seen in cross-section.
It is required that these rods are parallel and free of distortion, that the distances between the diagonally oppositely located rods are equal and that these diagonals form a right angle. These requirements are especially high for those mass filters which are to be used in higher mass ranges, i.e. masses that are higher than 500 atomic mass units (m&gt;500 u).
According to the equation ##EQU1## the ion mass m passed through the quadrupole filter is a function of the amplitude V and the angular frequency .omega. of the applied high frequency voltage, as well as of the distance of vertices 2r.sub.o of the respective rods. To ensure that the difference of the passed mass at any two points in the quadrupole filter with an adjusted passing mass m=1000 u is not larger than 0.1 u, the relative deviation of the distance of vertices ##EQU2## may be at most 1/20,000. In the case of a diagonal distance of vertices 2r.sub.o of usually 8 mm, this results in a required accuracy of 0.4 .mu.m.
For cylindrical rods with a two-point support, this value is already exceeded as a result of the natural bending under the influence of gravity. Accordingly, in such an arrangement of a multipole filter with pole rods and separate insulating holders, it is very difficult to meet the necessary accuracy requirements.
Therefore, the British Pat. No. 1,367,638 describes a filter which consists of a tubular, distortion-free and bending-resistant insulator with conductive surface coatings, wherein this filter is produced from an extruded ceramic tube with subsequent burning and partial coating of the inner surfaces with a conductive layer. However, the burning results in a shrinkage of the tube of approximately 10% and, therefore, does not meet the above-described requirements with respect to accuracy to size; accordingly, such a quadrupole filter is only used in the lower mass range as a residual gas analyzer.
The German Pat. No. 1,297,360 describes the production of highly precise glass tubes on a core with subsequent coating with metal of the indented inner surfaces to be used as a quadrupole system.
In this case, it is especially disadvantageous that the subsequent application of the conductive layer destroys the size retention ability of the analyzer tube. This is so because the quadrupole system, speaking in electrical terms, is a capacitor of the capacity C in which there is a high frequency voltage with the frequency .sqroot.=.omega./2.pi. and the amplitude V. With the usual data of C=50 pF, .sqroot.=2 MHz and V=5 kV, peak currents of i=V.omega.C=3 A are flowing. Currents of this magnitude flow through the conductive layer and cause a voltage drop over the layer between the various points of the surface in the quadrupole. Thus, according to the above equation for the distance of vertices 2r.sub.o and, considering the voltage V, a high precision must be demanded according to which the voltage at various points, under the conditions of the above stated numerical example, may at most deviate from its nominal value by 1/10,000, so that the resistance of one of the four conductive layers over its length may not exceed 0.1 .OMEGA.. In a metal having a specific resistance of 10.sup.-5 .OMEGA. cm which is used as a conductive layer metal, the length of the layer being approximately 20 cm and the width of the layer approximately 1 cm, a minimum thickness of the layer of 20 .mu.m must be demanded and, according to the above statements, the precision of the thickness of the layer must be within the range of 0.4 .mu.m . According to the present state of the art, such an accuracy cannot be achieved either electrolytically or by means of evaporating or coating.